feat(input): add the readonly styles for the ionic theme (#29483)

Issue number: internal

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## What is the current behavior?
There are no custom styles when readonly is added to an input.

## What is the new behavior?
- Applies an `input-readonly` class when the `readonly` property is
`true`
- Styles the class in the `ionic` theme to match the design requirements
for readonly inputs
- Adds screenshot tests for the readonly style for an unset `fill` and
`"outline"` fill

## Does this introduce a breaking change?

- [ ] Yes
- [x] No